In the casting of pigs, it is common to provide a pig sized for practically all sizes of pipes. The pipes are measured with their nominal diameter. It is common to pig fluid flow lines of sizes from about 4" and up so that the offering of a single pig design or shape requires a large number of molds in which to cast the pigs. In some instances, the pigs are made with a central body supporting mountable discs or cups. While this reduces the size of the mold, it still indicates that a specific mold size and shape is required to cast an individual cup or disc. Sometimes a large diameter mold can be made smaller by positioning a removable insert in it. In making a 16" disc, it is possible to reduce the diameter of the mod to 12", and perhaps even to 10" with an insert. However, this may require a more sophisticated insert in that reduction of the diameter changes the ratio of diameter to thickness. That ratio must be carefully observed to avoid departing from the design criteria which normally takes into account both diameter and thickness. As will be understood, the costs increase with an increase in diameter.
Common flow lines are found at 2" intervals. Therefore, beginning at 4", the nominal sizes of a set of pigs of a particular design should encompass the diameters of 4, 6, 8, 10, 12, etc. The measurements in inches again are the representative or nominal diameters. While the precise diameter or I.D. of the pipe may vary somewhat from the nominally stated dimension, the germane point of this discussion is that there is a required mold for each and every size. The molds become more expensive at greater sizes while the number of larger size pigs sold are reduced. Restated, there is simply a greater installed mileage base of pipelines measuring 12" and 16" compared to 32" or 36". To be sure, 36" pipelines exist but they require a mold which is much larger, they involve handling of much greater weights in molding and after molding and the unit sales do not correspond with the increased size of pipe diameter. Again, the most common pig sizes sold are typically 8, 12 and 16". The greater mileage of pipe of these dimensions requires a larger number of pigs of these sizes.
With the advent of a new type pig, capitalization cost must encompass the cost of making the molds. This can easily require a dozen molds to offer to the trade the different sizes of a newly devised pig. The present disclosure is directed to a method and apparatus for molding pigs which reduces the mold cost significantly. Indeed, through the present disclosure, pig cost can be significantly reduced and the capitalization cost can be reduced even more. This reduces storage space required for molds when not in use, and also reduces significantly the difficulties of handing such molds. As will be understood, a metal mold for making a 36" pig is quite heavy. In effect, it resembles a very large, heavy metal barrel and has to be handled with overhead hoists.
The present disclosure also sets forth an improved pig which is able to travel in both directions in a pipeline. For a pigging operation, removal and reversal of the pig can be done easily if there is a trap for the pig and if the pig is relatively small. When the pig is up to 12 or 16" in diameter, it can be handled easily. That is not so for a large pig. For instance, a pig measuring 24, 30 or 36" in diameter cannot be pulled easily from a pig trap reversed and reinserted for launching in a pipeline. It is necessary in many instances for a pig to travel in a bidirectional fashion. For instance, the pig may be launched from one location, sent to another location a few hundred feet or a few miles away, and then returned to the first location by reversing the flow. Pig travel in the pipe runs the risk of leakage around the pig. Leakage occurs as a result of flow by. There is another problem in handling the pig at the time of reversal. When the pig is exceedingly large and cannot be pulled from a pig trap and relaunched after reversal, it is important to have a pig that can run in both directions. Pigs made with cups simply cannot do this. Pigs made with discs can do this provided the cups do not deform. It is necessary to therefore flip the disc. By that, the bow in the disc must be reversed. So to speak, the frictional drag around the lip of the disc causes the surrounding edge to drag and fall behind the pig body. At the end of the line where a reversal in direction is required, the pig disc must snap, i.e., it must reverse or flip the peripheral edge to a trailing position. That is not easily accomplished in pigs. The present disclosure sets forth a pig which can accommodate these requirements. It is a pig which provides quality sealing in a pipeline. More than that, it is a bidirectional pig which is able to reverse movement without requiring intervention. The pig is simply reversed in its direction of movement and the discs in the pig are reversed. Such reversal is accomplished by the present pig taking into account the alternating hard and soft portions thereof. This pig construction enables reversal easily so that subsequent pig travel in the pipeline is readily done without difficulty.
In one aspect of the present invention, the pig is a two part polymeric foam system. The bulk or body of the pig is made of a soft foam. Several disks are integrally constructed along with a central core and they are made of a harder polymeric material. By forming the pig in alternate layers, improved sealing characteristics are obtained and the pig is able to flip, i.e., to move the peripheral edge to a trailing position with respect to the central core of the pig when reversed movement occurs.
In one aspect of the present disclosure, the pig is constructed so that there is better bonding of the components of the pig. One component is made of a light foam and is positioned adjacent to and serves as a mold piece for a heavier disc material. As the mold for the heavier material, better bonding is obtained between the two types of materials. This assures that the pig is able to hold together in a better fashion. This prevents breaking apart at the interface between different types of materials.
The present disclosure is therefore summarized as an improved method of manufacture of a molded pig formed of two parts or types of material. They are assembled by cutting donuts from a solid foam cylinder made of a very light weight foam. By use of surrounding straps, several donuts are joined in a single pour which defines the axial core and discs at spaced locations including the two ends of the pig. The several discs are defined at the same diameter to enable quality sealing in a pipe.